This application claims the priority of 8-231620, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an air flow meter for measuring an air flow rate, and more particularly to a heating resistor type air flow meter suitable for measuring an intake air flow rate of an internal combustion engine of a vehicle.
The conventional way of improving the measuring accuracy of a heating resistor type air flow meter used in an internal combustion engine under a pulsating flow condition, as disclosed in Japanese Patent Application Laid-Open No. 2-1518, provides a flow passage having an L-shaped detecting tube. That is, the flow passage comprises a wall against backward flow so that the back flow does not directly impinge on the heating resistor. Although such a flow passage construction cannot suppress back flow, it is possible to moderate a so-called binary-value phenomenon, that is, decrease of a detected value in the heating resistor type air flow meter which is caused when a pulsation amplitude of air flow increases.
Further, a flow passage construction having an orifice is disclosed in Japanese Patent Application Laid-Open No. 1-110220. In this construction, a heating resistor is arranged just downstream of an orifice inside a detecting tube which is a nearly straight and short tube parallel to the main flow direction.
In the prior art described above, it is impossible to measure flow speed by identifying direction of the flow. Therefore, when averaged output signals of the heating resistor type air flow meter are plotted as the boost pressure is being varied by gradually opening the throttle valve while rotating speed of the engine is kept constant, the averaged output signal gradually increases, but shows a jump-up phenomenon at boost pressures above a certain point indicating a plus side measuring error to an actual flow speed (flow rate), as shown in FIG. 12(b). The phenomenon is caused by the amplitude of pulsation of the heating resistor type air flow meter gradually increasing as opening degree of the throttle valve is increased and finally back flow occurs at opening degrees of the throttle valve above a point B, as shown in FIG. 12(b). The heating resistor type air flow meter cannot identify flow direction. Therefore, when back flow occurs, the averaged output increases because flow speed is equally detected independently of forward flow and back flow. It is known that this phenomenon often occurs particularly in an engine having four or less cylinders at a comparatively low rotating speed range of 1000 to 2000 rpm, and hardly occurs in an engine having more than four cylinders.
It is possible to reduce the error caused by back flow by employing one of the prior art teachings described above in which a wall against backward flow is provided in the flow passage so that the back flow does not directly impinge on the heating resistor. However, the error can be reduced by only a half. This is because when back flow occurs, forward flow increases by an amount of the back flow at the same time.
Further, it is difficult to prevent the back flow in an intake flow passage from occurring because of structures of the engine and the intake flow passage. Accordingly, in order to reduce the error caused by back flow, it is necessary to employ a complex method such as a structure in which an amount of back flow rate is subtracted from an amount of forward flow rate or a structure in which both of a forward flow rate and a back flow rate are separately measured.
An object of the present invention is to provide a low-cost and easy-to-handle heating resistor type flow meter by improving the measuring accuracy, including deviation accuracy, under pulsating flow accompanying back flow when the heating resistor type flow meter is mounted on a vehicle.
A heating resistor type flow meter to attain the above object comprises a main air flow passage body forming a main air flow passage for allowing a fluid to be measured flowing therethrough; and a measuring module having a heating resistor for measuring a flow rate of the fluid to be measured, inserted inside the main air flow passage body, wherein
the measuring module comprises the heating resistor inside an auxiliary shaped auxiliary air flow passage body forming an L-shaped auxiliary air flow passage having an inlet opening portion opening in a direction perpendicular to a main flow line of the fluid to be measured and an outlet opening an portion opening in a direction parallel to the main flow line; PA1 the main air flow passage body comprises an orifice on a periphery of the inner side wall positioned in an upstream side of the air flow passage body; and both of the inlet opening portion and the outlet opening portion are arranged within a flow flux zone formed by extending the fluid to be measured from a top edge of orifice in a direction parallel to the main flow line.
According to the present invention, because increase of flow speed within the flow flux zone formed by the orifice reduces an effect of back flow flowing in the auxiliary air flow passage body having the both opening portions arranged within the flow flux zone, the measuring accuracy can be improved.